In this historical photo from the U.S. space agency taken on Oct. 22, 1991, the HL-20, which was built at Langley in October 1990, and is a full-scale non-flying mockup, is shown. This mockup was used for engineering studies of maintainability of the vehicle, as testing crew positions, pilot visibility and other human factors considerations.

The HL-20 was a direct derivative of the HL-10 vehicle tested in the 1960s and bears a very close resemblance to engineering drawings produced at that time. Although evaluated as a possible "space taxi," the HL-20, sometimes called the "Personnel Launch System," was never built.

Each weekday, SPACE.com looks back at the history of spaceflight through photos (archive).